Cadmium and mercury in topsoils of Babagorogor watershed, western Iran: Distribution, relationship with soil characteristics and multivariate analysis of contamination sources

• Contamination sources of Cd and Hg in topsoils of the study area were analyzed.
• Cadmium and mercury concentrations are controlled by geological sources.
• High concentrations of Cd were found in soils originated from igneous rocks.
• Drainage network was likely the dominant factor controlling Hg variability.

The main objectives of this study were to identify spatial variability and main sources of cadmium and mercury in rural soils of Babagorgor watershed. A total of 87 composite soil samples were collected in an area of about 1352 km2. The average concentrations of the analyzed elements in topsoil were 3.289 mg Cd/kg and 0.632 mg Hg/kg. To identify the sources of heavy metals, we used: (a) the relationship between the heavy metal content in soils and the origin of the parent material; (b) the comparison of heavy metal in soils among different land uses; and (c) the heavy metal distribution in the study area. The results indicated that land use had no significant effect on Cd and Hg concentrations, and that the concentrations of Cd and Hg were primarily controlled by the parent materials of soils. High Cd concentrations in soils can be explained by occurrence of the magmatic rocks in the area. Mercury concentrations were associated with areas characterized geomorphologically as alluvial deposits at the lowest altitudes within the region. In fact, high adsorption of Hg with silt fraction allowed for its migration towards the drainage network.